More tests for String#hex, String#oct as against 9434506035

mrbgems/mruby-string-ext/test/string.rb

@@ -141,6 +141,7 @@ assert('String#hex') do
   assert_equal (-16), "-0x10".hex
   assert_equal 0, "xyz".hex
   assert_equal 16, "10z".hex
+  assert_equal 16, "1_0".hex
   assert_equal 0, "".hex
 end
 
@@ -151,6 +152,7 @@ assert('String#oct') do
   assert_equal 0, "9".oct
   assert_equal 0, "xyz".oct
   assert_equal 8, "10z".oct
+  assert_equal 8, "1_0".oct
   assert_equal 8, "010".oct
   assert_equal (-8), "-10".oct
 end